Because of their high power efficiency, pulse-width modulator (“PWM”) switching-mode buck converters are widely used in portable electronic equipment, such as cellular phones, MP3 players and digital cameras, to power digital signal processors (“DSP”) and other digital circuits. Buck converters step down the battery voltage and regulate the output voltage against variations from the battery and the load current.
There are various PWM-controlled buck converters used in power management applications. Because it requires fewer external components (e.g., compensation capacitors, resistors, and pins), has a fast response to the line and load disturbance, and uses a very simple design, conventional buck converter 100, as diagrammatically illustrated in FIG. 1A, is commonly used in many portable applications, such as cellular phones. Switch 105 of buck converter 100, conventionally implemented with PMOS, can be controlled by a fixed period and variable on-time control signal supplied to its gate from output 112 of comparator 110. Input voltage Vin is fed into the source of switch 105. Diode 135 is connected at a first end to input voltage Vin, opposite the source of switch 105, and at a second end to the drain of switch 105. Inductor (L) 130 is connected at a first end to the source of switch 105 and at a second end to a first end of capacitor (C) 140. Capacitor 140 is connected at a second end to input voltage Vin, opposite the source of switch 105. Resistor (R1) 115 is connected at a first end to the second end of inductor 130 and at a second end to resistor (R2) 120. Resistor 120 is connected at a second end to input voltage Vin, opposite the source of switch 105. Output voltage Vout (at the intersection of inductor 130, capacitor 140 and resistor 115) can be determined by the turn-on time of switch 105 and input voltage Vin. Output voltage Vout can be divided down by a resistor divider, including series resistors 115 and 120, taken from the intersection of resistors 115 and 120, and fed back to input 108 of comparator 110 as feedback voltage Vfb. Comparator 110 compares feedback voltage Vfb with ramp signal 125 (such as illustrated in FIG. 1B), which can be input to comparator 110 at input 106. Ramp signal 125, in the embodiment shown in FIG. 1B, has a constant frequency and fixed amplitude. When output voltage Vout varies, the duty-cycle of the output signal of comparator 110 will correspondingly change. Because the output signal of comparator 110 can be used to control the turn-on time of switch 105, output voltage Vout can be kept relatively constant. Buck converter 100 has a very fast response to the line and load change, but its line and load regulation ability is limited by its loop gain. Output voltage Vout, in continuous conduction mode (“CCM”), can be found as follows:
                                          V            out                    =                                                    V                ref                            +                              Δ                ⁢                                                                  ⁢                V                                                                                      R                  2                                                                      R                    1                                    +                                      R                    2                                                              +                                                Δ                  ⁢                                                                          ⁢                  V                                                  V                  in                                                                    ,                            Equation        ⁢                                  ⁢        1            where Vref and ΔV are voltages of ramp signal 125, as illustrated in FIG. 1B.
One approach to improve line and load regulation of buck converter 100, is to add loop 275, which has high gain, as shown in FIG. 2, to buck converter 100, forming buck converter 200. Loop 250 of buck converter 200 includes all the elements of buck converter 100 plus adder 225, into which feedback voltage Vfb is fed. Loop 275 can provide additional compensation for the variation in output voltage Vout Loop 275 includes a resistor divider, including resistor R3 215 in series with resistor R4 220, which is connected in parallel with series resistors 115 and 120. An end of resistor 215, opposite resistor 220, is connected to the first end of resistor 115, while an end of resistor 220, opposite resistor 215, is connected to the second end of resistor 120. Output voltage Vout can be divided down by the resistor divider of loop 275, including series resistors 215 and 220, taken from the intersection of resistors 215 and 220, and fed back to input 208 of comparator 210 as voltage 217. Reference voltage Vref is fed into comparator 210 at input 206. Output 212 of comparator 210 is connected to adder 225, thereby supplying signal 221 to adder 225. Adder 225 adds feedback voltage Vfb and signal 221 and provides the result to comparator 110, via input 108. Since loop 275 has high gain, capacitor (Cc) 255, connected between input 208 and output 212 of comparator 210, is needed to stabilize the loop as follows:Cc×(R3//R4)>>√{square root over (L×C.)}  Equation 2Capacitor 255 has a large capacitance. Therefore, if capacitor 255 is integrated on the control chip of buck converter 200, it will require a lot of silicon area. Conventionally, capacitor 255 is implemented externally, possibly requiring an extra pin from the control chip.
It is therefore desirable to provide a solution that improves the line and load regulation of a switching-mode power converter without requiring additional capacitors, either internally or externally, to stabilize the control loop. Exemplary embodiments of the present invention can provide this by integrating a compensator (or controller) with the PWM of the switching-mode power converter. Such a compensator can include comparators, digital circuits, and resistors.